Retinitis pigmentosa and age-related macular degeneration are two of the more frequent causes of blindness in the developed world (Bunker et al., 1984; Heckenlively et al., 1988; Friedman et al., 2004). Both diseases are progressive and begin with the degeneration of photoreceptors. In later stages of these diseases, bipolar, amacrine, and ganglion cells are still present, though their numbers are significantly decreased (Santos et al., 1997; Humayun et al., 1999a; Jones et al., 2005) and their spatial organization and circuitry is significantly disorganized (Marc and Jones, 2003). There are over 180 different gene mutations that result in photoreceptor diseases for which there is currently no cure or treatment {Daiger, 2007 #4484}. Ideally, it would be possible to develop a treatment for these conditions that would not require targeting each genetic defect independently.
It has been shown that retinal electrical stimulation in human patients during acute clinical testing results in the perception of bright punctate phosphenes (Humayun et al., 1996). Indeed, there are several groups developing implantable microelectronic visual prostheses that produce percepts by electrically stimulating the remaining retinal neurons. The ultimate goal of these projects is to generate useful vision in blind patients by transforming a video stream into a spatial and temporal sequence of electrical pulses that represents meaningful visual information. To date, several groups have succeeded in generating visual percepts via electrical stimulation with implanted acute, semi-acute, and long-term retinal prostheses in human patients (Humayun et al., 1999b; Rizzo et al., 2003; Weiland et al., 2004; Yanai et al., 2007; Zrenner, 2007). However, creating a perceptually meaningful pattern of stimulation is dependent upon a detailed understanding of the perceived intensity of any given stimulation pattern, and to date the literature examining the perceptual consequences of electrical stimulation remains relatively sparse (Humayun et al., 1996; Weiland et al., 1999; Humayun et al., 2003; Rizzo et al., 2003; Mahadevappa et al., 2005; Yanai et al., 2007).
A successful visual prosthesis needs to produce regions of constant brightness across a range of brightness levels, and ideally these brightness levels should be consistent with the apparent brightness of objects as they appear to those with normal vision. Our goal was to examine how apparent brightness changes as a function of stimulation intensity.
As described in earlier work, thresholds (the current required to reliably detect whether stimulation has occurred) vary widely across subjects and across electrodes {Mahadevappa, 2005 #4213; {de Balthasar, 2008 #4485}. These differences in threshold are likely due to individual differences between subjects and across the retina of individual subjects. Potential factors that may affect sensitivity to electrical current include the degree of retinal degeneration and possibly subject age, differences in degeneration (Marc and Jones, 2003) or sensitivity to electrical current across each subject's retina, and differences in the distance of the array from the retina {de Balthasar, 2008}. These factors are also likely to be associated with differences in apparent brightness as a function of stimulation amplitude.